Each tool has it's best tasks. Both are useful. Dremel sells replacement parts at reasonable prices. Home Depot used to carry some, may still. If not try LHS or mail order or Dremel online.
Start with a manual
http://www.dremel.com/en-us/customer...nd-literature/
2 speed multi-pro manual http://66.77.255.87/Images/DREMELMAN...ies_manual.pdf

Order parts from
Dremel Service Center Dremel Authorized Service
4915 21st Street
Racine, WI 53406
Phone: 1-800-437-3635

Switch P/N for 2 speed is 2610912829
Cost unknown.